It is well known that many sulfonate surfactants are useful in oil recovery processes, and that in such processes the surfactants are exposed to saline environments, particularly to concentrations of sodium and calcium salts. For example, aqueous surfactant solutions which are injected into subterranean oil-containing reservoirs often encounter deposits of high salinity brines. Furthermore, it is known that aqueous surfactant solutions undergo ion exchange with metal ions from reservoir clays. Still further, it is often desirable to use saline solutions, such as seawater or brines which have been recovered from the reservoir in conjunction with the oil, as sources of water for surfactant solutions injected into the reservoir for oil recovery purposes.
It is also recognized, however, that aqueous solutions of these as well as many other surfactants exhibit limited tolerance for dissolved salts, that is, the solubility of the surfactants in the solutions is significantly reduced as the concentration of salts in the solutions is increased. As a result, both the efficiency of the surfactants in oil recovery processes and the rate at which oil is recovered are reduced. Thus, in the case of processes which depend on the interfacial tension properties of the surfactant for direct mobilization of the oil, limited solubility of a surfactant means that it will move into and through the reservoir at a slower rate. Less surfactant will be available at a given location in the reservoir at a given point in time to mobilize the oil. In addition, limited surfactant solubility could convert a low interfacial tension middle-phase microemulsion to a higher interfacial tension upper-phase microemulsion which mobilizes less oil. In the case of processes which rely on mixtures of aqueous surfactant solutions with steam or other gases to generate "foams" which channel the oil recovery drive fluids to oil-rich areas of the reservoir, lower concentration of surfactant in the solutions results in a slower and less effective propagation of the foam front through the reservoir. After surfactant injection, the operator will encounter a longer delay in the onset of increased oil production.
In these and other cases, salt tolerance limitations may result in an effective loss of surfactant, an increase in process expense, and a decrease in oil recovery. For instance, a surfactant of limited salt tolerance may precipitate from solution as it encounters high salinity deposits in the reservoir. Quantities of surfactant above the limit of solubility fail to contribute to the oil recovery process.
It is the principal object of the present invention to increase the salt tolerance of sulfonate surfactants in aqueous solutions.